10.5.4 How Patterns Match

A target pattern is composed of a ‘%’ between a prefix and a suffix,
either or both of which may be empty. The pattern matches a file name only
if the file name starts with the prefix and ends with the suffix, without
overlap. The text between the prefix and the suffix is called the
stem. Thus, when the pattern ‘%.o’ matches the file name
test.o, the stem is ‘test’. The pattern rule prerequisites are
turned into actual file names by substituting the stem for the character
‘%’. Thus, if in the same example one of the prerequisites is written
as ‘%.c’, it expands to ‘test.c’.

When the target pattern does not contain a slash (and it usually does
not), directory names in the file names are removed from the file name
before it is compared with the target prefix and suffix. After the
comparison of the file name to the target pattern, the directory
names, along with the slash that ends them, are added on to the
prerequisite file names generated from the pattern rule’s prerequisite
patterns and the file name. The directories are ignored only for the
purpose of finding an implicit rule to use, not in the application of
that rule. Thus, ‘e%t’ matches the file name src/eat,
with ‘src/a’ as the stem. When prerequisites are turned into file
names, the directories from the stem are added at the front, while the
rest of the stem is substituted for the ‘%’. The stem
‘src/a’ with a prerequisite pattern ‘c%r’ gives the file name
src/car.

A pattern rule can be used to build a given file only if there is a
target pattern that matches the file name, and all
prerequisites in that rule either exist or can be built. The rules
you write take precedence over those that are built in. Note however,
that a rule whose prerequisites actually exist or are mentioned always
takes priority over a rule with prerequisites that must be made by
chaining other implicit rules.

It is possible that more than one pattern rule will meet these
criteria. In that case, make will choose the rule with the
shortest stem (that is, the pattern that matches most specifically).
If more than one pattern rule has the shortest stem, make will
choose the first one found in the makefile.

This algorithm results in more specific rules being preferred over
more generic ones; for example:

Given these rules and asked to build bar.o where both
bar.c and bar.f exist, make will choose the first
rule and compile bar.c into bar.o. In the same
situation where bar.c does not exist, then make will
choose the second rule and compile bar.f into bar.o.

If make is asked to build lib/bar.o and both
lib/bar.c and lib/bar.f exist, then the third rule will
be chosen since the stem for this rule (‘bar’) is shorter than
the stem for the first rule (‘lib/bar’). If lib/bar.c
does not exist then the third rule is not eligible and the second rule
will be used, even though the stem is longer.